why do sportsbooks sometimes refund bets on suspicious activity

Had a couple parlays get refunded last month and it always throws me off when it happens. They say it’s due to "suspicious activity" or "irregular betting patterns," but I honestly never get the exact reason. Makes me wonder what actually counts as suspicious on their end. Is it big swings in odds or like one guy loading the same bet over and over? Or are they just worried about insider info? I don’t see this in table games - roulette never gets voided for “suspicious spins.” Why do they only do it for certain sports bets?